Lockbolt Qualification Testing for Wing Panel Assemblies

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2000-01-3023
Lockbolt Qualification Testing for Wing Panel Assemblies
Samuel O. Smith
Electroimpact, Inc., Mukilteo, WA
Gary Potticary, Gareth Lewis
Airbus UK Limited, Filton, UK
Copyright © 2000 Society of Automotive Engineers, Inc.
ABSTRACT
This paper gives an outline of testing carried out in
conjunction with Electroimpact to support the introduction
of the A319/A320/A321 and A340-500/600 Panel
Assembly Cells in Broughton, UK. Testing compared the
percussion insert/EMR swaging of lockbolts with existing
hydraulic installation methods. Tests included pre-load
tension tests, ultimate tension load tests, tension fatigue
tests, high-load lap shear fatigue tests, static lap shear
tests, a pressure leak test, and metallurgical
examination.
Fastener configurations tested covered diameters from
1/4, 5/16, 3/8, and 7/16 of an inch. Joint materials
conformed to ABM3-1031 (7150-T651 plate), stump-type
lockbolts to ABS0550VHK (Huck LGPS4SCV), and
collars to ASNA2025 (Huck 3SLC-C). Some pull-type
lockbolts to ABS0548VHK (Huck LGPL4SCV) were also
tested as noted.
INTRODUCTION
Airbus UK Limited manufactures wings for the Airbus
family of large commercial aircraft at their Broughton
facility in the U.K. It is desirable and practical to have
automated CNC machines install the lockbolts between
stiffeners and panels for such aircraft. Please see
Figure 1. When a new process was proposed for
installing lockbolts at the facility, testing was needed to
assure that the new process would provide a joint
performance that meets or exceeds existing processes.
ı Prior (baseline) processes: hydraulically pulled
(manual pin-tail) or pushed (Drivmatic) lockbolts
with hydraulically swaged collars.
ı New (now current) process: pneumatically
percussion driven lockbolts with EMR swaged
collars.
Low-voltage EMRs have been developed to install
lockbolts by Electroimpact since 1991. (Please see
reference 1.) For production aircraft, test requirements
were defined by the Materials and Process Group of
Airbus UK Limited, Filton. As part of the risk share
agreement, Electroimpact performed testing of the
percussion insert/EMR swage of lockbolts. Material
specimens, fasteners, and collars were provided by the
airframe manufacturer. Joint materials conformed to
ABM3-1031 (7150-T651 plate), stump-type lockbolts to
ABS0550VHK (Huck LGPS4SCV), and collars to
ASNA2025 (Huck 3SLC-C). Some pull-type lockbolts to
ABS0548VHK (Huck LGPL4SCV) were also tested as
noted. This test work verified the integrity of the
electromagnetic installation process, helped set voltages
of the EMRs for collar swaging, and extended the
process for lockbolt diameters up to 7/16 inch.
Figure 1. CNC Machine Fastening of Stiffeners to Panels
Unless noted, lockbolts for these tests were installed with
a pneumatic percussion tool. Some pull-type lockbolts
were installed with a hydraulic installation tool. EMR
(electromagnetic riveter) swaging of the lockbolt collars
was done on a test bench as similar to Figure 2. Fatigue
testing of specimens was done on the author’s MTS
fatigue tester, see Figure 3. Fatigue coupons were also
assembled for the airframe manufacturer’s Technical
Centre as laboratory controls.
Figure 2. Test Bench for EMR Collar Swaging
Testing was carried out to verify pre-load, ultimate tensile
strength, tension-tension fatigue, high-load lap shear
fatigue, static lap shear, fuel retention and metalluragical
properties.
Figure 3. Fatigue Testing Machine.
TESTS PERFORMED ON LOCKBOLTS
A series of tests were used to verify the quality of the
pneumatically driven, EMR swaged lockbolts. Although
all tests are needed to verify different characteristics of
the installed lockbolt/collar combination, special
emphasis was placed on the pre-load and fatigue
specimens.
PRE-LOAD TESTS
When a collar is swaged on the serrated end of a
lockbolt, the entire shank of the lockbolt is put in tension.
The amount of tension is known as “pre-load” and is a
critical parameter for ensuring the integrity and the
fatigue life of a bolted joint. Tests were performed for
determining the optimum voltage setting range for 1/4,
5/16, 3/8 and 7/16 inch nominal diameter lockbolt collars.
Tests were per MIL-STD-1312-16 as shown in Figure 4
below.
Inserting a lockbolt and swaging a collar between two
thimbles with clearance fit holes as shown in Figure 4
makes the pre-load test specimens. A thin friction
paddle is sandwiched between the two thimbles. There
is hole through the center of both thimbles and the
paddle. The ends of the thimbles are installed in a
tensile test machine and axially loaded at a known rate,
such as 6700 N/min (1500 pound/min). The paddle
moves when the axial force matches the pre-load
tension. Further, the axial load is increased until the
collar fails in tension. The aluminum of the collar is
sheared in the lockbolt grooves as shown in Figure 5.
Figure 4. MIL-STD-1312-16 Pre-load Test Schematic
Figure 5. Several Pre-load Specimens after Testing
A series of pre-load test was performed for each fastener
diameter. The voltage setting of the EMR on the collar
side controls the amount of the swaging force. A range
of acceptable EMR swaging voltages was found for each
fastener diameter. Table 1 below lists the fastener
manufacturer’s minimum pre-load tension, minimum
ultimate load, and the airframe manufacture’s load for
tension fatigue test.
lockbolt
diameter
Minimum
pre-load
Min. tensile
ultimate
Tension
fatigue load
in N (lbf.) N (lbf.) N (lbf.)
1/4 6670
(1500)
13300
(3000)
5560
(1250)
5/16 11100
(2500)
22200
(5000)
8450
(1900)
3/8 15600
(3500)
31100
(7000)
13300
(3000)
7/16 22200
(5000)
44500
(10000)
17800
(4000)
Table 1. Lockbolt Tension Targets and Loads
Figures 6 through 9 show the relationship between
swaging voltage and pre-load. Taking Figure 7, for
example, the minimum pre-load tension is 11,100 N
(2,500 lbf.) and the minimum ultimate load is 22,200 N
(5000 lbf.) Collar side voltages from 235 to 275 volts
yield acceptable pre-loads for these 5/16 inch lockbolts.
In production, acceptable pre-load for the minimum and
maximum stack thickness for a given grip narrows the
actual range of voltages used to approximately ±10 volts.
EMR charging voltage is controlled within ±1.5 volts. This
testing also identified a need to change the philosophy
for establishing the swage gauge design.
1/4 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
150 160 170 180 190 200 210 220
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Huck
HG113
fail
Huck
HG113
pass
Figure 6. ¼ Collar Pre-load Tests
5/16 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
6000
7000
200 210 220 230 240 250 260 270 280
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Huck
HG113
fail
Huck
HG113
pass
Figure 7. 5/16 Collar Pre-load Tests
3/8 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
250 260 270 280 290 300 310
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Figure 8. 3/8 Collar Pre-load Tests
7/16 collar preload tests, 75% voltage on head versus collar
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
280 290 300 310 320 330 340
collar side swaging voltage, V
preload tensile
ultimate tensile load
minimum
ultimate
minimum
preload
Figure 9. 7/16 Collar Pre-load Tests
TENSION-TENSION COLLAR FATIGUE TESTS
Tension-tension fatigue of lockbolt collars was performed
to MIL-STD-1312 test 11. Cyclic tension is put on the
lockbolt head and the swaged collar to the loads listed in
the far right column of Table 1. Twelve collars were
EMR swaged, three for each diameter 1/4”, 5/16”, 3/8”
and 7/16”. All tests were stopped after 130,000+ cycles
without failure. Collars were then tested to ultimate load.
All tests exceeded the minimum required ultimate load of
the collar. Tension fatigue life is a function of good
fastener design. These tests again verified that the EMR
collar swaging process provided adequate performance
compared to existing processes.
HIGH-LOAD LAP SHEAR FATIGUE TESTS
These tests compared the lap shear fatigue life of 1/4-14
lockbolts with EMR installed collars to the existing
hydraulic squeeze (Drivmatic) process. The test
coupons conformed to the airframe manufacturer’s
drawings. In addition, Electroimpact tested additional
coupons with two bolt holes as shown in Figure 10. See
Figure 11 for a photograph of a specimen in the fatigue
test fixture. In practice, the jaws (plates) clamping the
coupon were shimmed to center and parallel within 0.12
mm (0.005 inch.)
These coupons were subjected to a net alternating
stress of 100, 150, or 210 MPa (14.5, 21.7, or 30.5 ksi)
with a mean stress of zero. (Fully reversing tension and
compression loads). Over 30 fatigue tests were
performed by Electroimpact. 16 tests with specimens
provided by the airframe manufacturer. 20 preliminary
tests were also carried by Electroimpact with 7075-T6
aluminum coupon material.
Figure 10 High Load Lap Shear Fatigue Test Coupon
Figure 11. Fatigue Test Fixture – Loaded High Lap Shear Coupon
A comparison of the fatigue results for hydraulically
pulled and swaged lockbolts versus pneumatically
driven, EMR swaged lockbolts is given in Figure 12. For
example, at a stress level of ± 150 MPa (21.7 ksi), both
the hydraulically swaged and the EMR swaged
specimens cracked around 100,000 cycles. Similar
results can be seen at the higher and lower stress levels.
Control specimens and further tests conducted by
Airbus UK Limited at Filton verified the integrity of joints
assembled by this process.
STATIC LAP SHEAR TESTS
These tests were carried out to verify the static shear
strength of lockbolts with electromagnetic set collars.
The specimens were two rectangular aluminum plates
with a two lockbolts through the lap joint to the airframe
manufacturer’s drawings. The plates are pulled, causing
shear stresses in the lockbolts. Laboratory control
coupons were provided to the airframe manufacturer. In
addition, two coupons with 7075-T6 material were tested.
Both coupons failed by shearing the (1/4 inch nominal)
titanium alloy lockbolts at 50.7 kN (11,000 lb.) and 50.5
kN (11,350 lb.) respectively. The EMR swaging process
provided good static lap shear strength.
PRESSURE FUEL RETENTION TESTS
A pressure chamber test was used to verify the fuel
retention capability of pneumatically driven/EMR swaged
lockbolts. This test was per MIL-STD-1312-19. Airbus
UK Limited provided the test plate as shown in Figure 13.
Electroimpact applied sealant to the countersink,
pneumatically installed the lockbolts, and EMR swaged
the collars. Huck International performed the test at their
Irvine, California test facility. Pressure in the test
chamber was cycled to 50 psi for 1000 cycles. After the
successful test, Figure 14 illustrates the top of plate with
bolt heads under die penetrant developer.
METALLURGICAL EXAMINATION
Electroimpact provided (14) specimens to the airframe
manufacturer’s Technical Centre for metallurgical
examination. Lockbolt sizes were 1/4-4, 1/4-15 and
5/16-7. Typical microsections are shown in Figures 15
and 16 in the following illustrations. Lockbolts were
pneumatically/percussively driven and EMR swaged.
Specimens were cut from the plates, potted in polymer,
faced, polished, and photographed under a microscope.
Microsections verify the integrity of EMR swaged collars.
CONCLUSIONS
Pneumatically driving and EMR swaging lockbolt collars
met or exceeded the current Drivmatic (hydraulic)
installation in terms of pre-load, lap shear fatigue life,
and static shear strength. These and the other airframe
manufacturer’s tests provided confidence to invest in
equipment for the electromagnetic swaging of lockbolts.
Pre-load tests also identified a need to change the
swage gauge design philosophy when using this type of
process. A wide range of acceptable voltage settings can
be used for EMR swaging. In addition, these tests give
design engineers confidence that the allowable loads,
strength, and fatigue life of these installed fasteners
exceed the lockbolt manufacturer’s minimum values.
Electromagnetic riveters (EMRs) have now installed
millions of lockbolts in production on large commercial
aircraft. They have been used on lockbolts from 1/4 to
7/16 inch diameter.
ACKNOWLEDGMENTS
The authors acknowledge Airbus UK Limited, Filton
Design Centre for allowing the publishing of this test
information The authors also wish to acknowledge the
assistance of Huck International by providing fasteners
and test assistance.
REFERENCES
1. “Low Voltage Electromagnetic Lockbolt Installation”,
John Hartmann, et.al., SAE Aerofast 1992 paper
922406
CONTACT
For more information, contact Sam Smith at
Electroimpact, Inc, 4606 107th St. SW, Mukilteo, WA,
98275, telephone 425-348-8090, fax 425-348-0716. Email
sams@electroimpact.com
DEFINITIONS, ABBREVIATIONS
CNC: computer numerically controlled, any machine or
process controlled by a computer, where the computer
controls the machine position and processes.
Collar: a cylinder (typically of aluminum alloy) that is cold
formed on the serrated end of a lockbolt pin.
EMR: electromagnetic riveter. An electromechanical
device that installs and forms aerospace fasteners.
Grip: the range of joint thickness that a fastener can
assemble. For instance, a stump type lockbolt with a
–11 grip can fasten a stack thickness from 11/16 inch
(17.5 mm) down to 10/16 inch (15.9 mm).
Lockbolt: A two piece fastening system consisting of a
headed parallel shank pin (generally titanium alloy) and a
collar (generally alloy aluminum) swaged on to the
serrated pin end.
Figure 12. Results – High Load Lap Shear, Hydraulic versus EMR Swage
Figure 13. Fuel Tightness test – Collar Side of Test Plate with Slotted Packing Ring
Figure 14. Fuel Tightness Test – Top of Plate with Bolt Heads Under Penetrant Developer
Figure 15. Typical Sectioned Lockbolt and Collar, 5/16-7 Shown
Figure 16. Microsection of Collar Groove Fill, Approximately 50x Scale
titanium bolt
grooves
aluminum
collar

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